scsi: MAINTAINERS: Add mpi3mr driver maintainers

Adding mpi3mr driver entry.

Link: https://lore.kernel.org/r/20210623072153.25758-1-sreekanth.reddy@broadcom.com


Signed-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>